This image made from Jan. 11 video released by the U.S. military shows an airstrike targeting an Islamic State group cash and finance distribution center near Mosul, Iraq. The extremist group that once bragged about minting its own currency is having a hard time meeting expenses as a result of coalition airstrikes, plummeting oil prices and a cutoff of payments from the Iraqi government, the sum of which has eroded millions from its finances since last fall. | COMBINED JOINT TASK FORCE-OPERATION INHERENT RESOLVE VIA AP

A series of U.S. airstrikes in recent months have destroyed “hundreds of millions” of dollars in Islamic State cash as part of a broader campaign aimed at squeezing the extremist group’s financial power, a U.S. military spokesman said Wednesday.

Col. Steve Warren, speaking for the coalition that is fighting the Islamic State in Iraq and Syria, said the effort — which has included airstrikes against oil processing and distribution facilities mainly in Syria — has forced the Islamic State to adjust by reducing salaries for its fighters.

The Associated Press reported on Tuesday that the Islamic State is having a hard time meeting expenses. It reported that the group has slashed salaries across the region, asked residents of Raqqa, its de facto capital in Syria, to pay utility bills in black market American dollars, and is now releasing detainees for a price of $500 a person.

Warren said that in some cases Islamic State has reduced salaries to fighters by as much as 50 percent.

“This, to us, is one indicator that these strikes against their ability to generate revenue are beginning to squeeze them a little bit,” he said.

Warren had previously estimated that strikes against Islamic State cash holdings had eliminated tens of millions of dollars. On Wednesday, he said the latest estimate is hundreds of millions, but he would not offer a more specific total.

Of a total of 10 U.S. airstrikes against Islamic State cash holdings since October, seven have been in Iraq and three in Syria. The first was Oct. 21, followed by other single strikes on Nov. 14, Dec. 16, Jan. 11 and Jan. 18. The campaign was intensified last weekend with a series of five airstrikes near Mosul, in northern Iraq, hitting what Warren described as two Islamic State financial distribution centers and two financial storage centers.

Separately, the Pentagon said Wednesday that the air campaign against Islamic State has cost a total of $6.2 billion since it began in August 2014. The daily cost is now averaging $11.5 million.

Coalition aircraft struck 10 more of the cash collection points over the weekend, said Warren.

“We don’t have a hard number that we’re prepared to release. We believe it is in the hundreds of millions of dollars,” Warren told Pentagon reporters in a video conference from Baghdad.

“Obviously it is impossible to burn up every single bill. So presumably they were able to collect a little bit of it back. But we believe it was a significant series of strikes that have put a real dent in their wallet,” he said.

Targeting the cash hoards is part of a broader strategy to disrupt the group’s sources of revenue.

Coalition aircraft also have been hitting oil infrastructure under Islamic State control as part of the effort.